1. Field of the Invention
The present invention relates to end effectors used in robotics and other applications, and more particularly to reconfigurable end effectors having a plurality of digits mounted for rotational and pivoting movement to accomplish the grasping of objects of various different shapes.
2. Description of Related Art
End effectors, sometimes referred to as mechanical hands or robotic hands, are employed for a wide range of applications where mechanical manipulation is required. In particular, virtually any industrial or other application of robotics requires an end effector of some type to provide a manipulation capability. Accordingly, a wide variety of different types of end effectors are known in the art and a great deal of effort has gone into their design and development.
There are various types of end effectors including those designed for specific industrial applications and those which are application non-specific and are reconfigurable to adapt to various tasks. Application non-specific end effectors are typically designed to provide a high degree of reconfigurability and manipulation capability. Such effectors are typically complex, have a large degree of freedom and employ complex actuation mechanisms. They may attempt to emulate the human hand, and are often referred to as robotic hands. Typically, robotic hands employ three or four digits, with each digit having two or three degrees of freedom. Examples of robotic hands include the Utah-MIT dextrous hand described in Machine Design (Jun. 26, 1986), page 40, the so-called Salisbury hand described in Robots, Philip de Ste. Croix, ed. (Salamander Books 1985), page 53, and the reconfigurable hand manufactured by Victory Enterprises Technology, Inc. of Austin, Tex.
A further example of a reconfigurable end effector or robotic hand is described in co-pending application Ser. No. 07/512,042 now U.S. Pat. No. 5,108,140 of Bartholet, RECONFIGURABLE END EFFECTOR, filed Apr. 17, 1990 and commonly assigned with the present application. The end effector described in the Bartholet application employs three digits, each having two pivoting degrees of freedom and a rotational degree of freedom. The digits have an asymmetrical configuration, with two of them rotating about an offset axis and a third rotating about a non-offset axis, allowing the digits to reconfigure about a payload. Each digit includes an actuation mechanism for driving the pivoting degrees of freedom, and including a differential transmission mechanism therein in conjunction with a double parallelogram configuration formed by the moving segments. Such arrangement provides a selective enveloping or parallel vice grip grasping action.
Although end effectors such as that described in the Bartholet application have various advantages over previous end effectors so as to be capable of functioning in a highly advantageous and effective manner for many applications, there is room for further improvements in such devices. Typical of such devices is a digit actuation mechanism having a motor directly coupled through a speed reduction mechanism of fixed gear ratio and a clutch/brake mechanism to a drive member such as a lead screw to provide the pivoting action of the digit. The actuation mechanism must be capable of handling high mode loads encountered by the digit, while at the same time being capable of moving the digit at a reasonable speed to the different desired positions thereof. This typically requires a relatively large motor, which makes it even more difficult to design a small end effector with adequate power.
Sensing of the digits to determine their positions and load conditions in presently known end effectors typically requires relatively complex arrangements of strain gauges and other instruments, due in part to the need to constantly monitor the gripping and the rotational positions of each digit independently. Momentary malfunctioning of the control system can result in loss of registration. In some cases the striking of another digit or immovable object is not sensed in time, and damage to the digit results.
Where the digits are operated by lead screws, lead screws of the circulating ball type are often required in order to minimize friction, but at considerable expense. Back driving of the lead screw can also be a problem, and typically a brake is required to prevent this from happening. Typical arrangements for rotating the digits are somewhat inefficient, and some require energy wasteful active devices to lock the digits in their desired rotational positions. Registration and tracking of digit rotation can be difficult. Certain arrangements of the prior art end effectors require two motors per digit, one to rotate the digit and the other to provide pivoting movement of the various segments of the digit.
It would therefore be advantageous to provide a reconfigurable end effector which overcomes the disadvantages of prior art end effectors. Among other things, each digit would desirably be operated by a single motor of relatively small size. The actuation mechanism for the digit should preferably be of economical construction and capable of preventing back drive without the need for separate braking apparatus. It would also be desirable to provide an actuation mechanism which simplifies the sensing operation.